An ice cube that sinks
Ordinary ice floats in water. But an ice cube made of “heavy water” sinks to the bottom of a glass of ordinary water. Heavy water looks exactly like regular water and is also made of hydrogen and oxygen. So why is it heavier? The secret is inside the nucleus of its hydrogen atoms.
Isotopes
Atoms of the same element that have the same number of protons but different numbers of neutrons are called isotopes同位素. The name comes from Greek for “same place”: isotopes occupy the same place on the periodic table. (You’ll study the periodic table in lesson 1.4.2; for now, it’s enough to know that each element has one box in it, as in the lookup table in lesson 1.3.3.)
Hydrogen has three isotopes:
| Name | Symbol | Protons | Neutrons | Mass number |
|---|---|---|---|---|
| Protium (ordinary hydrogen) | 1 | 0 | 1 | |
| Deuterium (heavy hydrogen, symbol D) | 1 | 1 | 2 | |
| Tritium (symbol T) | 1 | 2 | 3 |
Heavy water is water made with deuterium instead of ordinary hydrogen (). Each molecule is a bit heavier than a normal water molecule, so heavy-water ice is denser than ordinary liquid water.
Some other common isotopes:
- Carbon: , , . Carbon-12 is the standard for atomic masses (lesson 1.3.5), and radioactive carbon-14 is used to date ancient objects.
- Chlorine: and .
- Uranium: is the fuel in nuclear power plants.
One element can have several isotopes. 118 elements are known, but more than 3000 different kinds of atoms (counting every isotope) have been identified.
Properties of isotopes
Isotopes of an element have the same number of protons and therefore the same number of electrons. An atom’s chemical properties depend mainly on its electrons, so the isotopes of an element behave almost identically in chemical reactions: both and react with sodium to form sodium chloride.
What differs is mass. Different numbers of neutrons mean different atomic masses, so substances made from different isotopes have slightly different physical properties, such as density, just like heavy water and ordinary water.
Build some isotopes
Add and remove neutrons and watch whether the element changes and whether the nucleus is stable. Build the isotopes of hydrogen and carbon:
Not to scale: a real nucleus is only about 1/100,000 the diameter of its atom.
| Particle | Protons | Neutrons | Electrons | Mass number |
|---|---|---|---|---|
| hydrogen-3 | ||||
| carbon-14 | ||||
| oxygen-18 | ||||
| chlorine-35 | ||||
| chlorine-37 |
Example 1: Comparing two chlorine atoms
Compare and : protons, neutrons, electrons, and chemical properties.
- Protons. Both have atomic number 17, so both have 17 protons. They’re the same element.
- Neutrons. : 35 − 17 = 18. : 37 − 17 = 20. The numbers of neutrons differ.
- Electrons. Both are neutral atoms, so electrons = protons = 17.
- Conclusion. They are isotopes. With the same electrons, their chemical properties are nearly identical; their masses differ.
Example 2: Same mass number, so isotopes?
and both have a mass number of 40. Are they isotopes?
- Compare protons. Argon has 18 protons and calcium has 20.
- Identify the elements. Different numbers of protons mean different elements.
- Conclusion. They are not isotopes. Isotopes must be the same element, with the same number of protons. Matching mass numbers mean nothing here.
- and
Both are carbon (6 protons), with 6 and 8 neutrons.
- and
Different numbers of protons, so different elements. The matching mass number doesn't matter.
- and
Both are chlorine, with 18 and 20 neutrons.
- and
Both are hydrogen, with 0 and 2 neutrons.
- and
Isotopes are atoms. and are two different molecules (elemental forms) of oxygen, not isotopes.
- and
One is nitrogen (7 protons) and the other is carbon (6 protons): two different elements.
- and
Both are uranium (92 protons), with 143 and 146 neutrons.
- and
These are two molecules (compounds), not atoms. Only the H and D inside them are isotopes.
Common mistakes
Mistake 1: “Isotopes are different elements.” Isotopes have the same number of protons, so they’re different atoms of the same element. Protium, deuterium, and tritium are all hydrogen.
Mistake 2: “Atoms with the same mass number are isotopes.” Isotopes have the same number of protons and different numbers of neutrons. and share a mass number but are different elements.
Mistake 3: “Isotopes behave very differently in reactions.” Isotopes have the same electrons, so their chemistry is nearly identical. What differs is mainly their mass, and whether the nucleus is radioactive.
Quick check
What is different among atoms of , , and ?
All three are carbon, with 6 protons (nuclear charge +6) and 6 electrons. Only the neutrons differ: 6, 7, and 8.
Which pair are isotopes of each other?
Isotopes are atoms of the same element with different numbers of neutrons. A and C are substances (molecules), and D is two different elements.
Summary
- Atoms of the same element with the same number of protons but different numbers of neutrons are isotopes. They share one place on the periodic table.
- Hydrogen has three isotopes: protium, deuterium, and tritium. Carbon-14 is used for dating; uranium-235 is a nuclear fuel.
- Isotopes have the same electrons, so their chemical properties are nearly identical; their masses, and so some physical properties, differ slightly.
- To decide whether two particles are isotopes: check that they’re atoms, then that the protons match and the neutrons don’t.
On to AP (for later)
In AP topic 1.2, mass spectrometry separates an element’s isotopes by mass. Each peak in a mass spectrum is one isotope, and the height of the peak shows how abundant it is. In the next lesson you’ll use those abundances to calculate an element’s average atomic mass.